Field of the Invention
The present invention relates to a wafer boat and a manufacturing method for the same, and relates to a wafer boat which holds a silicon wafer in a vertical low-pressure CVD apparatus used in a manufacturing process for a semiconductor device, for example, and a manufacturing method for such a wafer boat.
Description of the Related Art
A CVD apparatus is employed for forming a film by chemical vapor deposition on a surface of a silicon wafer to be processed. FIG. 4 illustrates a conventional vertical low-pressure CVD apparatus 30. The CVD apparatus 30 includes a furnace main body 31, a process tube 32, accommodated in the furnace main body 31, in which a plurality of silicon wafers W can be loaded, and a heater (not shown) disposed between the furnace main body 31 and the process tube 32. The process tube 32 is made of high-purity quartz or silicon carbide (SiC), and can maintain a high-temperature state by being heated on the inside. The process tube 32 is connected to a vacuum pump (not shown), to reduce an internal pressure less than or equal to a predetermined pressure (for example, 1.3 kPa).
A central part of a base 33 to be covered with the process tube 32 is provided with a boat receiver 34 on which a vertical rack type wafer boat 1 is disposed. The wafer boat 1 holds the plurality of silicon wafers W with a predetermined spacing in the vertical direction. On one side of the wafer boat 1 are provided a gas inlet tube 35 and a thermocouple protection tube 36. The gas inlet tube 35 introduces reaction gas into the furnace. The thermocouple protection tube 36 incorporates a thermocouple for measuring the temperature in the furnace.
In such a vertical low-pressure CVD apparatus 30, the plurality of silicon wafers W is held on the wafer boat 1 and housed in the furnace main body 31.
Next, after the pressure of the furnace is reduced to a predetermined pressure of 1.3 kPa or less, for example, and the temperature in the furnace is raised to a temperature between 600° C. and 900° C., for example, a polycrystalline silicon film and a silicon nitride film, for example, are formed on the silicon wafer surface by introducing a reactive gas (material gas), such as SiH4, with a carrier gas such as hydrogen gas H2 into the furnace through the inlet tube 35.
The conventional wafer boat 1 is disclosed in, for example, JP 2008-277781 A. The wafer boat 1 disclosed in JP 2008-277781 A includes, as illustrated in FIG. 5, a pair of a top plate 3 and a bottom plate 4 on the upper and lower sides, each plate having a larger outer diameter than the silicon wafer W to be held, and a plurality of (three in FIG. 5) columns 2 connecting between the plates. The top plate 3 and the bottom plate 4 are formed to have a disc-shape similar to the silicon wafer W.
As illustrated in FIG. 6, the column 2 is provided with support grooves 2a for supporting the silicon wafers W, which forms shelf plate portions 2b projecting from the side surface of the column. The upper surface of the shelf plate portion 2b serves as a supporting plane 2b1. That is to say, the silicon wafer W is held by placing an outer peripheral portion of the silicon wafer W on each supporting plane 2b1 of the column 2.
The wafer boat 1 is made of SiC-based material with a surface thereof provided with a SiC coating film formed by CVD processing. The SiC coating film suppresses diffusion of impurities from the inside of the base material to the outside.
Incidentally, when the wafers W in the wafer boat 1 are subjected to CVD processing, a processed film (hereinafter referred to as a deposit film) is deposited not just on the wafers W but also on the wafer boat 1 that holds the wafers W.
The CVD processing may produce a difference in thermal expansion coefficient between the deposit film (such as SiN or Si) and the SiC coating film formed on the surface of the base material of the wafer boat 1. This difference results in the lack of adhesion strength of the deposit film attached on the SiC coating film (an anchor effect), which may cause adhesion to the silicon wafer W of the deposit film as particles which is peeled off from the SiC coating film, when loading wafers W on the shelf plate portion 2b. 